Both the U.S. Bureau of Public Roads and the state of Nevada urged the inclusion of the cutoff route into each state's highway systems, the former as part of the federal aid highway connecting Salt Lake City and Los Angeles,[12] and the California state legislature did that in 1925,[13] with it becoming an extension of Route 31. (Across the state line in Nevada, State Route 6 continued through Las Vegas to Arizona.) The initial plan for the U.S. Highway system simply stated that Route No. 91 would run from Las Vegas "to an intersection with Route No. 60" (which became US 66 in 1926),[3] but in 1926 the cutoff was chosen, ending at US 66 at Daggett, just east of Barstow.[4][14]

The original routing south from Las Vegas to Needles later became part of US 95 in 1940. The new "cutoff route" was added to the federal-aid secondary system in 1926,[15] which helped pay for a mid-1930s widening and paving, including some realignments (parts of the old road are now known as Arrowhead Trail). The new routing generally followed the present I-15, except through Baker (where it used Baker Boulevard) and into Barstow (where it followed former SR 58 to First Avenue, ending at Main Street, which carried US 66).[16] It entered San Bernardino on Cajon Boulevard, then followed the route of Arrow Highway between San Bernardino and Los Angeles.